The Absolute Write Water Cooler is a huge moderated forum that covers all aspects of writing. Of particular interest is the Bewares and Background Check section. Here members can ask about agents and publishers and get feedback from other forum members on their experiences with them (good and bad). This feedback is generally open and honest and reflects the real world position. If you need information on a specific agency - or agents within an agency - here's the place to come.
In addition, most of the big scam agencies are identified so newbies can find out who to avoid when sending out submissions.
As a US site there is an understandably US bias to most of the entries but most of the big UK agencies/publishers are covered as well.
Highly recommended for those starting out on the submissions trail.